1. Field of the Invention
This invention relates generally to a vent damper which is positioned in the vent passageway of a furnace or similar combustion device and blocks off the vent to prevent the passage of heated air therethrough when the furnace is not in operation. More particularly the invention relates to such a damper actuated by an automatic mechanism having an electric motor which closes the damper, a spring which opposes the motor to open the damper, and an over-travel mechanism which causes the damper to stop in its full open condition while the gear train connecting the damper, the motor, and the spring continues to idle to a stop.
2. Prior Art
The field of automatically operated vent dampers has been of increased interest in the past several years due to the rising cost and scarcity of heating fuel. When a furnace is in operation its chimney or flue must be open to the outside atmosphere so that the combustion products may exhaust outside the building. When the furnace shuts off, however, there is no longer any need for the vent to be open and, in fact, it is undesirable for it to remain so. Heated air in the furnace and in the room in which the furnace is located will tend to rise up out of the open vent resulting in significant heat loss to the building.
Closing off the vent when the furnace is not in use is the obvious solution and there are many devices commercially available that can be retrofitted to vents that will accomplish this. Many of these automatic devices use a torsion spring to open the damper and an opposing electric motor which is actuated when the furnace is not operating to close the damper and hold it in a closed position. The damper, the spring, and the motor are connected by a gear train such that as the motor closes the damper it also winds up the spring. When the motor is shut off the spring uncoils to open the damper and in doing so it spins the motor shaft in the direction opposite its drive direction until the damper reaches an end stop defining its open position.
Certain problems, however, arise when the exact requirements of such a device are noted. For instance, the vent damper must be fail-safe; i.e., noxious exhaust fumes and/or heating gas will back-up into the furnace room if the vent is closed while the furnace is operating, so the damper must be designed so that no mechanical or electrical malfunction will allow this. Thus, a relatively strong opening force is called for to insure that any friction or binding forces on the moving parts will be overcome.
Weighing against this requirement, though, is the consideration that the gear train interconnecting the damper, the motor, and the spring is relatively delicate and will not withstand much in the way of an impact such as can be caused when a moving gear train is stopped suddenly. As a powerful torsion spring drives the damper to an open position it is subjecting the gear train to a substantial amount of torque. In currently used vent dampers, when the damper reaches the end of is travel towards the open position and contacts an end stop this torque is transferred to the gear train as an impact force. Particularly sensitive to this impact is the small pinion gear which characteristically connects the motor to the gear train.
Consequently it has been necessary (1) to moderate the strength of the torsion spring in order to reduce stress and wear on the gear train and (2) to try to machine and assemble all the parts to tolerances exact enough to keep friction and binding down to a level that the spring can be relied upon to overcome when opening the damper. Due to the hostile environment that a vent damper mechanism must contend with operating in a chimney, however, it is very difficult to construct a mechanism that can maintain the narrow balance between two powerful a spring, which will damage the gear train, and too weak a spring, which will not be able to overcome the friction and drag that will generally increase as the damper becomes dirty, corroded, and worn during its service life.
The graph of FIG. 6 is a plot of damper position--ranging from fully open to fully closed--versus the torque provided by the torsion spring. Line M indicates the torque path for the conventional spring return mechanism. Point A gives the torque contained in the spring in its fully wound position and point B the torque contained in the spring when the damper has reached its end stop and the spring has stopped unwinding. This torque path is ideal in that it is high enough through the entire range to insure that the damper will open, and low enough so that when the damper reaches its open position excessive strain is not placed on the gear. Lines H and L indicate respectively the highest and lowest variations of the torque path about the mean that are allowable in a vent damper control mechanism. If Line H is exceeded an unacceptable amount of stress as applied to the gear train when the damper reaches its end stop and if Line L is not exceeded the spring torque is not sufficient to insure full opening of the damper. These tolerances must be maintained for proper operation of the vent damper, but current manufacturing techniques cannot consistently do so.
The results of the foregoing engineering conflict is that vent dampers currently on the market are expensive to manufacture to the exacting tolerance required and even then are less than totally reliable and have a short service life.